1. Field of the Invention
The present invention relates to a printing apparatus having ultrasonic actuators serving as power sources for reciprocating a carriage and feeding a sheet.
2. Related Background Art
Conventionally, an apparatus having a construction shown in FIG. 2 has been known as a thermal jet type printing apparatus employing ultrasonic actuators as power sources.
In FIG. 2, referential numerals 5 and 8 denote typical ultrasonic actuators. As shown in FIG. 3, the ultrasonic actuator 5 consists of a radial fin type elastic member 12 having a pair of linear portions and a pair of arcuate portions and a piezoelectric element 13 fixed to the elastic member. When the piezoelectric element 13 is applied with plural kinds of AC voltage having phases electrically different from each other, in order to generate travelling vibration over the surface of the elastic member 12, a carriage 19 including a printing head 4 is shifted by a slider 1 which is in contact with the surface of the elastic member 12. A linear guide 7 guides the carriage 19.
A sheet 11 on which printing is performed is transferred by the ultrasonic actuator 8, which is the same as the ultrasonic actuator 5. In fact, as shown in FIG. 4, both sides of the sheet 11 are appropriately pressure-welded, that is, pinched by a pair of ultrasonic actuators 8 and 8'. In FIG. 4, crests of both travelling waves generated over the elastic member surfaces of respective ultrasonic actuators 8 and 8' concur with each other, and the travelling waves are controlled to travel to the same direction with respect to the sheet 11. At this time, specific mass points on the elastic member surfaces of the actuators more elliptically as 8a and 8'a in the figure. Due to such elliptical motion, the sheet 11 is transferred toward the direction opposite to that of the travelling waves.
A support plate 3 supports the upper actuator 8, which is one of the ultrasonic actuators 8 and 8' for feeding the sheet. A rotary encoder 9 which is connected with a roller 9a pressure-welded onto the sheet and is rotated therewith detects the shifting amount of the sheet. A sheet guide 10 along which the sheet is slid and carried to prevent the sheet from slanting. A linear encoder 6 for the carriage optically detect the shifting amount and the position of the carriage to determine timing for the printing head 4 to discharge ink. A home position sensor 2 is used to determine the absolute position of the printing head from the home position, wherein the carriage is generally moved to the position of the home position sensor when power is applied. The count value of the linear encoder is cleared at that position, and after that, the position of the carriage including the printing head is regulated relatively on the basis of the value detected by the linear encoder 6.
FIG. 5 shows the control circuit for the ultrasonic actuator(s).
In the figure, an oscillator 14 generates pulses according to the multitude of DC voltage, a ring counter 16 determines one of outputs .o slashed..sub.1 to .o slashed..sub.4 to be switched on in turn according to the output of the oscillator 14 serving as a clock, switching transistors 17a to 17d perform switch-on/off operation according to the output of the ring counter 16, and a center tap type transformers 18a and 18b generates increased secondary AC waves according to the switch-on/off operation performed by the switching transistors. The phases of the outputs of the transistors 17a and 17b, as well as those of the outputs of the transistors 17c and 17d, are shifted from each other by 180.degree., while the phases of the transistors 17a and 17c, as well as those of the transistors 17b and 17d, are shifted from each other by 90.degree.. Accordingly, the transformers 18a and 18b output secondary AC waves having phases shifted from each other by 90.degree.. As understood from the above construction, the frequency of the secondary outputs from the transformers is 1/4 of the frequency of the oscillator 14. Ultrasonic actuators 5, 8 and 8' are driven by applying two kinds of properly increased voltage having two phases shifted from each other by 90.degree.. Though only one ultrasonic actuator is shown in FIG. 5, actually three actuators are provided in the printer shown in FIG. 2, wherein they are used to drive the carriage and feed the sheet. The three actuators may be controlled by using corresponding three circuits similar to that shown in the figure. Otherwise, they may be controlled by switching the outputs of the circuit shown above. An encoder 20 corresponds to the linear encoder 6 and the rotary encoder 9 shown in FIG. 2. On the basis of the output of this encoder 20, in order to obtain stable speed, the value of the frequency designated to the oscillator 14 is controlled by a controller 15, which comprises, for example, a microcomputer. FIG. 6 shows relation of shifting speed of the carriage to the frequency of driving voltage applied to the ultrasonic actuator 5 for driving the carriage. In FIG. 6, fr is the resonance frequency of the ultrasonic actuator, at which the shifting speed of the carriage is maximal. As is clearly understood from the graph, as the driving frequency decreases from 46 kHz, the speed of the carriage gradually increases. Below fr, however, the speed of the carriage suddenly decreases. That is, because of the reversion from increase to decrease of the speed of the carriage at fr with respect to decrease of the frequency, the frequency should be always higher than fr in order to smoothly control the speed.
The conventional printing apparatus having the above-mentioned construction has the following problems.
As the resonance frequency is not always the same but varies depending on changes in characteristics such as temperature and other environment factors, the characteristic curve shown in FIG. 6 may shift horizontally. Therefore, the speed varies even when the carriage is driven at the same frequency, and may suddenly decrease when the resonance frequency becomes higher than the driving frequency.
In order to always obtain stable operation of the printer regardless of the above-mentioned characteristics, it is necessary to detect the characteristics of the speed with respect to the driving frequency under the operational environment in advance. When operation to detect the characteristics is performed independent of actual print operation by the printer, however, loss of time or unnatural print operation may occur. An additional circuit for knowing the characteristics would increase cost.